The present invention relates to a method of fabricating a semiconductor laser device, and more particularly, to a method of fabricating a semiconductor laser device in which catastrophic optical damage (COD) is suppressed.
An exemplary method of fabricating a semiconductor laser device will be shown below.
First, a lower cladding layer made, for example, of n-AlGaAs, a lower optical confinement layer made, for example, of non-doped AlGaAs, an active layer made, for example, of InGaAs/GaAs in a multi-quantum well structure, an upper optical confinement layer made, for example, of non-doped AlGaAs, an upper cladding layer made, for example, of p-AlGaAs, and a cap layer made, for example, of p-GaAs are laminated on a semiconductor substrate made, for example, of n-GaAs by an epitaxial crystal growth method such as an MOCVD method in this order to fabricate a slab laminated structure.
Then, the top surface of the laminated structure is formed into a ridge shape. Subsequently, Ti/Pt/Au, for example, are vapor deposited on the ridge-shaped upper surface to form an upper electrode (p-type electrode) for making an ohmic contact with the cap layer. Also, on the bottom surface of the semiconductor substrate, AuGeNi/Au, for example, are vapor deposited to form a lower electrode (n-type electrode).
Then, the laminated structure is cleaved to have a predetermined cavity length, thereby forming facets of a laser cavity. Generally, a lower reflective coating is formed on one facet, and a higher reflective coating on the other facet, using a dielectric material such as a silicon nitride, to fabricate an intended laser device.
Conventionally, high optical power semiconductor laser devices have been used in a variety of fields such as optical communication systems, optical information recording on an optical disk and so on, laser printers and laser machining, solid-state laser excitation, optical sources for wavelength conversion such as SHG, and so on. In recent years, the demand for higher power of the semiconductor laser devices has increased substantially.
For example, in the field of optical communication systems, an optical amplifier (EDFA) fabricated using erbium doped fiber (EDF) was brought into practical use in the early 1990""s. At the beginning of practice, the power required to a semiconductor laser device for use as an exciting optical source for exciting the EDFA was at most several tens of milliwatt (mW). However, with the dramatic advance of the recent wavelength division multiplex (WDM) technology and so on, semiconductor laser devices for use as exciting optical sources have been required to provide high power well exceeding 100 mW. In addition, the semiconductor laser devices are required to simultaneously have the driving reliability of driving for about 1,000,000 hours even at such high power.
Among factors which limit the high reliability of semiconductor laser devices, the most critical factor is the catastrophic optical damage (often called xe2x80x9cCODxe2x80x9d) on the facets of a cavity. The COD results from positive feedback based on repetitions of a sequence of processes which involve absorption of light on the facets of the cavity; a temperature rise on the facets; a contraction of the band gap of a semiconductor material which comprises an active layer on the facets due to the increase of temperature; and an increase in the amount of absorbed light at the facet due to the contraction of the band gap.
The most effective expedient for suppressing the occurrence of such COD is to epitaxially grow a crystallized semiconductor material which has a band gap larger than that of the semiconductor material comprising the active layer. This epitaxially grown material is often referred to as the xe2x80x9clarge band gap semiconductor materialxe2x80x9d.
For example, the inventors"" Japanese Patent Application Publication No. 8-32167-A discloses a method which involves forming an upper electrode and a lower electrode in the aforementioned laminated laser structure, cleaving the structure to define the laser""s facets, and growing a large band gap semiconductor material as mentioned above on a cleaved plane of the structure. As such, in this method, the large band gap semiconductor material grown on the facets is also deposited on the previously formed electrodes. Therefore, the growth of the large band gap semiconductor material on the facets is preferably done in a low-temperature environment (for example, in a range of 300xc2x0 C. to 500(copyright) C.) to avoid any abnormal reactions between the large band gap semiconductor material and the material of the electrodes.
While this approach has worked for the inventors, they have observed that the thus treated facets sometimes have increased optical absorption characteristics for some types of laser devices, particularly those having material layers which contain aluminum (Al). The inventors have discovered that the increased absorption characteristics can reduce the lifetime of the laser diode and present obstacles to developing lasers with higher output power. The present invention is focused on reducing these undesirable optical absorption characteristics, while still reducing the occurrence of COD.
In the course of making their invention, the inventors have found there is oftentimes a thin but highly rigid film of aluminum oxide present at the laser facets for those laser diodes which are comprised of one or more layers having aluminum (Al) therein. This aluminum oxide layer is located at the cleaved surface, between the laminated laser structure and the epitaxial layer of the large band gap semiconductor material. The inventors have found that this aluminum oxide forms relatively quickly in the time between the cleaving operation and the epitaxial growth operation, even though this time is very short. The inventors have further discovered that the thin layer of aluminum oxide causes an abnormal growth condition for the large band gap semiconductor material, or any other type of semiconductor material grown thereon, which creates defects in the crystal structure of the grown material. In addition, the inventors have found that these defects increase the optical absorption at the laser""s facets. The inventors have further found that attempts to reduce the aluminum oxide to aluminum by heating the device to temperatures above 500xc2x0 C. or by exposing the device to a chemical reducing agent have not been practical with the present state of technology.
Broadly stated, the present invention provides a method of fabricating a semiconductor laser device which comprises the steps of cleaving a laminated structure comprised of semiconductor materials to form facets of a laser diode cavity, and epitaxially growing a semiconductor material on the facets of the laser diode cavity, wherein the step of cleaving and the step of epitaxially growing are performed in a low oxygen and moisture concentration atmosphere.
In some preferred embodiments of the present invention, the semiconductor material, which may be a compound semiconductor material, has a band gap which is larger than the bandgap of at least one of the layers of the laminated structure, and is preferably larger than the band gaps of a plurality of the layers of the laminated structure, particularly the active layers and the optical confinement layers.
In some of these preferred embodiments and still other preferred embodiments of the present invention, the laminated structure of the laser diode is maintained within a low oxygen concentration and low moisture concentration environment from the time of the end of the cleaving operation to the time of the start of the epitaxial growth operation.
Accordingly, it is an object of the present invention to reduce or eliminate undesirable optical absorption characteristics at the facets of a laser diode.
It is another object of the present invention to provide a method of fabricating a semiconductor laser device which increases the optical output power of the device.
It is still another object of the present invention to provide a method of fabricating a semiconductor laser device which increases the lifetime of the device.
It is object of some preferred embodiments of the present invention to provide one or more of the forgoing benefits while reducing or eliminating the occurrence of catastrophic optical destruction.
These and other objects of the present invention will become apparent to one of ordinary skill in the art in reviewing the present specification and attached claims.